1. Field of the Invention
This invention relates to a resist for alkali development which is useful for a fine working technique in the manufacture for example of a semiconductor element and the like.
2. Description of the Related Art
In the manufacture of electronic parts such as an LSI, a fine working technique utilizing a photolithography has been employed. In this fine working technique, a resist film is first formed by coating a resist solution onto a semiconductor substrate. The resist film thus formed is then exposed to light through a prescribed mask pattern, and subsequently subjected to various treatments including an alkali development, thereby forming a resist pattern. This resist pattern is then utilized as an etching resistant mask in a dry etching process, and the exposed surface portions of the substrate are etched away to form a pattern of fine lines or openings. Finally, the resist pattern remaining on the substrate is removed through ashing.
Therefore, the resist to be employed in this process is generally required to have a high dry-etching resistance. In view of this requirement, a resist comprising an aromatic compound has been extensively employed. For example, many kinds of resists comprising an alkali-soluble novolak resin as a base resin have been developed up to date.
In view of a trend in recent years to further increase the integration of semiconductor elements such as seen in an LSI, the aforementioned fine working technique in recent years has made it possible to realize a fine pattern of the order of sub-half micron. It is expected that this trend to further increase the fineness of pattern will be further promoted in future. In order to meet such a trend, the use of light source of shorter wavelength in photolithography is now being studied. For example, a method for forming a finer resist pattern through the use of ArF excimer laser (193 nm in wavelength) or the quintuple harmonic of YAG laser (218 nm in wavelength) has been tried at present.
However, the aforementioned conventional resist comprising a novolak resin as a base resin generally exhibits a tendency to absorb the aforementioned light of short wavelength to a large extent at the benzene nucleus of the novolak resin. Therefore, it is impossible, because of this large light absorption ratio, to allow the exposure light to reach deep enough to an interface between the resist film and the substrate, thus making it very difficult to form in high sensitivity a fine pattern which is excellent in shape and in precision.
Since the resist comprising a novolak resin as a base resin is insufficient in transparency to the light of short wavelength as mentioned above in spite of its excellent dry-etching resistance and its alkali-developing properties, there has been a strong demand for the development of a new resist which is suited for use in a photolithography where the ArF excimer laser or the quintuple harmonic of YAG laser is employed.
In view of this, a resist where an alicyclic compound is employed in place of an aromatic compound is recently attracting attention. For example, a resist comprising a polymer having an adamantane side chain as a base resin is proposed in Japanese Patent Unexamined Publication Hei/4-39665, the resin being excellent in dry etching resistance and in transparency to a light of short wavelength. There is also disclosed in this prior art an example where a compound having an adamantane side chain is copolymerized with an acrylic compound having a carboxylic group to provide a copolymer having an alkali-solubility so as to form a resist pattern by way of an alkali development.
However, there are various problems in an alkali development of a resist pattern by the use of resist comprising an alicyclic compound, because the alkali-solubility of an alicyclic structure such as the adamantane differs greatly from that of carboxylic group. For example, the solubility and elimination of a predetermined region of a resist film becomes non-uniform at the occasion of development, thereby inviting not only the deterioration of resolution, but also the generation of cracks or surface roughening due to partial dissolution of a resist film portion which should have been kept unremoved. Moreover, an alkali solution may penetrate into an interface between the resist film and the substrate, thereby causing a peeling of the resist pattern. Additionally, a phase separation between the alicyclic structure portion and the carboxylic group portion may be readily proceeded, thus making it difficult to prepare a homogeneous resist solution and deteriorating the coating properties of the resist solution.